Optical fibers, and especially single mode fibers, are widely used in telecommunications. These are highly sensitive to microbends of the order of hundreds of microns which may be the result of pressure by grains of contaminants in the fiber coating or due to contractions of the fiber due to temperature changes. The fibers are also sensitive to "macro-bends" of the order of bend radii of some millimeters, which are encountered when an optical fiber is wound on a small diameter support. Such bends occur during the production of a wide variety of products, such as pressure sensors of hydrophones, of gyros, etc. Typical products comprise a plurality of windings on a support of the order of from about 5 to 20 mm diameter. For many other uses other diameters or shapes of bends are used. The sensitivity of fibers to bends is highly variable for different optical fibers, depending amongst others on the construction of the fiber, its profile, geometry, MFD value. the wave-length propagated via the fiber. One known method is based on the winding of a fiber from a large drum onto one having a small diameter, and measuring signal attenuation due to the winding onto the small one. A system for measuring optical attenutation in optical fibers is set out in U.S. Pat. No. 4,996,420 where the bent region is propagated along the fiber and signal attentuation is measured.